Gas lasers are usually fabricated using one of two methods with respect to alignment. Both methods utilize a tilt or sideways alignment of the optical cavity comprised of the mirrors in order to align it with the gain tube in the laser. In a first method of passive coarse and fine alignment, the laser cavity is coarse aligned by the use of simple optical alignment methods such as autocollomation. Then the cavity is fine aligned by the use of the cavity as an interferometer, using an external monochromatic light source (e.g. a laser). The cavity is then sealed using, for example, optical contact or epoxy. The laser is then evacuated and filled. In a second method of passive coarse alignment and active fine alignment, the laser cavity is coarse aligned as in the first method. The sealing can be accomplished with several techniques such as optical contact, epoxy or glass frit. The laser is evacuated and filled. Fine alignment is next accomplished by deforming the support of one or both mirrors, for example, by using a metal tube with necked down section using set screws to accomplish the necessary adjustment while operating the laser. The laser is fine aligned to give maximum output power.
The disadvantage of the first method of passive coarse and fine alignment is that it is performed prior to activating the laser. This is necessary because the laser cannot be operated until the seal is fully cured. After the seal is made and the laser activated, it is not possible to realign the mirror. Therefore, the alignment is limited by the instability of the seal during its curing process. Passive alignment using an external light source is also more difficult than direct measurement of the light output of the activated laser. And since it is an indirect method, it is likely to be less accurate.
The disadvantage of the second method of passive coarse alignment and active fine alignment is chiefly high cost and lack of mechanical stability. The lack of mechanical stability arises from the fact that the metal in the necked down section, usually made of fully annealed Kovar, has to be plastically deformed. It is also difficult to finely align the mirror with set screws having a coarse thread compared with the desired fine alignment that usually has to be done within a few arc seconds tilt of the mirror. There is also a safety hazard in adjusting the metal part since it is generally used as the anode for operating the laser. The adjusting screws will be at a high voltage potential.
The main disadvantage with epoxy seals is due to outgassing and water permeability. both of these characterisitcs tend to limit a laser's life length. Consequently, there is a movement away from this type of seal in the manufacture of lasers. Another problem that affects an epoxy seal is temperature limitation. Epoxy usually cannot be heated higher than approximately 110.degree. C. before decomposition. Strain and shrinkage due to complete curing affect the mirrors. There is also a gross mismatch in thermal expansion that affects this type of seal.
Although there is a movement in the field of lasers to hard (i.e. hard vacuum) seals and, more specifically, toward glass frit seals, it does not come without problems. High temperature, approximately 300.degree. C., is necessary to make the seal. This seal has to be made in air, because of two reasons. First, the mirror degrades both by dissociation of oxygen from the coating when heated in oxygen deprived atmosphere and by crazing of the coating. Second, the glass frit usually contains lead oxides that reduce to free lead if heated without oxygen. Thus, no active fine alignment with the laser operating is possible, at least not by sliding the mirror while heating it. Also, a frit seal needs quite exact match of thermal expansions, further limiting the choice of the glass frit. Usually the glass frit has bad flow properties making even a coarse alignment a problem. Also, most solder glasses devitrify after heating for a short time, making yield and adjustment a problem. The glass frit seal doesn't bond to the mirror coatings. Therefore, it is necessary to either restrict the coated surface or remove it by, for instance, sand blasting from the seal area.
Optical contact seals are very expensive and can, therefore, be used only on very special lasers such as ring laser gyroscopes, where the added cost is relatively smaller than for ordinary gas lasers. Optical contact seals are expensive because of the requirements of good surface finish, scratch and dig 10--5, and requirement of flatness, .lambda./10. Contact cannot be made on curved surfaces. It is further not possible to align the laser actively, for instance, by sliding on a curved mirror back and forth because of leaks and because the vacuum inside the laser would make the mirror stick onto its contact surface.
None of the seals described above afford the laser manufacturer the possibility to readjust a laser cavity. Because these seals do not provide a means for readjusting the laser cavity, the yield in production is low. It is, therefore, necessary when using these seals to combine anyone of the above seals with a metal bellows or metal tube with a necked down section in order to accomplish fine adjustment.